Fluorescent lamp ballast



Dec. 20, 1960 w. w. BROOKS ETAL 2,965,799

FLUORESCENT LAMP BALLAST Filed Sept. .26, 1957 |a,l4 M

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INVENTORS 3 17. 3 WESLEY W. BROOKS,

CHARLES E-STRECKER BY W ATTORNEY United States Patent '0 FLUORESCENT LAMP BALLAST Wesley W. Brooks and Charles E. Strecker, Danville,

11]., assignors to General Electric Company, a corporation of New York Filed Sept. 26, 1957, Ser. No. 686,307

13 Claims. (Cl. 315-97) Our invention relates to apparatus for starting and operating fluorescent lamps, now commonly known as ballasts, and more particularly to such ballasting apparatus for hot cathode type fluorescent lamps and to high reactance transformers for such apparatus.

Recently developed hot cathode type fluorescent lamps, such as 6 or 8 foot rapid start lamps and grooved lamps operate on starting voltages and lamp currents considerably above those formerly produced. Marketing requirements, however, dictate that the ballasting apparatus for these newly developed lamps should be as small or smaller than prior ballasts, particularly in the height dimension. These prior ballasts, such as those described and claimed in US. Patent 2,796,554-Strecker, are quite satisfactory when encased in housings above 2% inches high and when intended for operating lamps with currents below 0.425 ampere; but they result in undesirar' bly high transformer leakage reactance and poor lamp current wave form when crowded into shallower housings or when designed to deliver higher lamp currents or unusually high starting voltages.

The usual engineering expedients for reducing transformer leakage reactance, such as the location of some of the secondary winding turns over or alongside the primary winding either reduce the open circuit voltage coupling factor of the more loosely coupled secondary turns of the transformer or distort the lamp current wave form or have both undesired effects.

Accordingly, an important object of the invention is to provide ballasting apparatus for high current fluorescent lamps in which the ballasting transformer may be contained within a shallow housing and yet have a desired low leakage reactance and a high coupling factor.

Another important object is to provide a smaller, more easily produced, high reactance transformer for high current fluorescent lamps in which each half of the transformer is of substantially identical construction, and electrically and magnetically symmetrical relative to its primary winding.

A further object of the invention is to provide ballasting apparatus for a plurality of high current hot cathode type fluorescent lamps, which apparatus operates the lamps with high light output and improved lamp current wave form.

In general, in accord with the invention a ballasting apparatus is provided having a high leakage reactance transformer in which the secondary winding is divided into two serially connected secondary winding sections having balanced leakage reactances and located on opposite sides of the primary winding. The core of the transformer contains two bridged gaps each located in the magnetic circuit of a different one of the secondary winding sections. Each half of the transformer on either side of its central crosssectional plane is preferably substantially identical. A capacitor is preferably connected lice power factor. When used with hot cathode type lamps, filament heating windings are provided concentric with or adjacent to the primary winding.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may be easily understood by referring to the following description taken in connection with the accompanying drawing in which;

Figure l is a plan view of the ballasting apparatus of the invention with bottom cover removed but without the usual potting compound and without the electrical connections to and from the various coils and other components of the apparatus;

Figure 2 is a schematic circuit diagram showing the electrical circuit connections to and from the components of the apparatus of Figure 1 when connected to operate a pair of fluorescent lamps; while Figure 3 is a vectorial diagram of the operating voltages in the circuit of Figure 2.

Referring to Figure 1, the general construction of one form of applicants invention for operating two hot cathode type fluorescent lamps is shown as comprising a ballasting apparatus 10 contained within a housing 11. The ballasting apparatus 10 includes a high reactance transformer 12; an operating or running capacitor 13 and a starting capacitor 14 located Within a single case 15; a pair of capacitor discharging resistors 16 and 17 across the terminals of capacitors 13 and 14 respectively; and a radio interference suppressing capacitor 18.

High reactance transformer 12 has a shell type magnetic core 19 made up of a central elongated winding core leg 20 and a pair of outer yoke core legs 21, 22 in respective magnetic circuit relation with the central winding core leg 20. End clamps 23, 24 serve securely to fasten the winding and yoke core legs together in assembled relation.

A primary winding P, a plurality of heating windings H H and H and a pair of secondary windings S and S are located on the central winding core leg 29 within the window space 25 provided between the central and yoke core legs of the transformer 12. The primary winding P and the heater windings H H and H are located on the central portion of winding core leg 20 and are preferably concentric with one another. Secondary winding sections S and S are located on opposite sides of the primary winding P. Secondary winding sections S and S are preferably of substantially identical construction and symmetrically arranged on the core relative to primary winding P.

The magnetic circuits of the secondary winding sections S and S each contain a non-magnetic gap bridged by a magnetic flux restricting or saturating path or section, such magnetic core arrangements being now generally known as bridged gaps In the embodiment of the invention shown in Figure 1 such bridged gaps 26 and 27 are located in the central core leg 20, bridged gap 26 being located in the region of core leg 20 traversed by secondary winding section S and bridged gap 27 being located in the region of core leg 20 traversed by secondary winding section 5 Bridged gaps 26 and 27 are preferably located equidistant from the opposite sides of the primary winding P, as shown.

It will thus be seen that each half of high reactance transformer 12 on either side of its central transverse cross-sectional plane 30 may be of identical construction, as shown. It will be appreciated that several manufacturing conveniences and economies may be effected because of this symmetrical construction. For example, the yoke core legs 21, 22 may be substituted for each other and the laminations of these yoke core legs as Well as the laminations of central core leg 20 may be reversed endnuances.

for-end. The secondary winding sections S and S may, likewise be substituted for one another while the relative concentric location of the primary and heating windings may be varied on'the central portion ofwinding core leg 20,

Referring to Figure 2 the electrical connections of the ballasting apparatus 10 of Figure l are shown, the ap paratus 10 being connected to operate a pair of fluorescent. lamps 31 and 32' of the type having heated filamentary electrodes 33, 34, 35 and 36. The ballasting apparatus 10 contains a pair of input leads 40, 41 connected to opposite ends of the primary winding P and three pairs of output leads 42, 43; 44, 45; and 46, 47, each pair of output leads being connected to a respective one of the heater secondary windings H H and H Output leads 42, 43 are connected across lamp electrode 33; output leads 46, 47 are connected across lamp electrode 3.6; while output leads 44 and 45 are connected through conductors 48, 49 across both lamp electrodes 34 and 35.

Within ballast housing 11 the secondary winding sections S and S of transformer 12 are connected in series with one another through conductor 50 and in autotransformer relation with the primary winding P through conductor 51. Secondary winding sections S and S are wound in voltage aiding relation with each other and with the primary winding as indicated by arrows 52. Running capacitor 13 is connected between output lead 42 and the end 53 of the secondary winding opposite to that connected to the primary winding lead 40 and is thus in series circuit relation with the autotransformer 12 and the lamps 31, 32, the other side of the primary winding P being directly connected through conductor,

54 to output,lead.47 thereby to complete this series circuit. Startingcapacitor 14 is connected withinapparatus 10' between output 1eads 42 and 44 and is thus in parallel circuit relationwith. one ofthe fluorescent lamps 31.

Capacitor di'scharging resistors 16 and 17 of high'ohmic. resistance are respectively connected in parallel with ca-1 pacitors 13 and 14. Radio frequency suppressing capacitor 18 is connected between output leads 42 and 47 and is thus in parallel circuit relation with both fluorescent lamps 31 and 32.

In the operation of ballasting apparatus 10, input leads 40 and 41 are connected across a suitable source of alternating voltagesof power frequency, for. example, 115

volts at 60 cycles. Alternating output voltages for heat ing lamp electrodes33, 34, 35am! 36 are, immediately developed by heating windings H H and. H of transformer'12and ares'upplied tothe lamp electrodes through. theoutput' leads42-47. Simultaneously, a much higher.

voltage" is developed across the primary and secondary windings ofthe autotransformer 12 between input lead.

41 and the upper end 53 of its secondary winding S This autotransformer voltage is immediately applied across fluorescent lamp 32 through capacitors 13 and 14 and output leads 44 and 47, the turns ratio of the autotransformer being constructed to develop an open circuit voltage great enough to start this lamp 32. As soon as lamp 32 starts, a voltage is built up across starting capacitor 14 which is substantially equal to the autotransformer open circuit voltage, and this voltage is applied across lamp 31 through output leads 42 and 44 thereby to start this second lamp 31. Once lamp 31 fires, the starting capacitor 14 is effectively by-passed by the conduction of this lamp 31. Once both lamps 31 and 32 have fired, the running circuit includes the autotransformer 12, capacitor 13 and the fluorescent lamps 31, 32 all connected in series across an alternating voltage source applied to input leads'40, 41. In this series cir} cuit'the inductive react'ance of high reactancetranse former 12 and thecap'acitive reactance of capacitor 13 function to "provide the ballasting impedance for. the.

lamps.

A vectorial diagram .of typical voltages produced across the components of this circuitis showninFigure 3. As will be seen from this, diagram. the. magnitude. of the open circuit voltage across secondary winding sections S and S is unusually close to the magnitude of the running voltages developed across the secondary winding sections. For example, it is not unusual with the construction of applicants invention to obtain an open circuit-to-run voltage ratio for the secondary winding sections above 0.7. This is to be contrasted with prior arrangements where the secondary winding is located only on one side of the primary winding or a portion thereof wound concentric with the, primary winding in which case the ratio of secondary winding open circuitto-running voltage is. usuallyless than 0.6.

As mentioned above. the division of the secondary winding coil into two secondary winding sections located on opposite sides of the primary winding together with the provision of bridged air gaps in the magnetic circuit of each of the winding, sections enables the achievement of the desired high. open, circuit voltage for the transformer with relatively low leakage reactance, good coupling factor between the primary and secondary windings, and a relatively low level of distortion in the lamp current wave form as well as good balance between the light output provided by thetwo lamps.

As a specific example, a ballast constructed in accord with the invention for igniting two 72T12/RS type lamps had the following outer casing 11 dimensionswidth 3 /8 height 2 /8", and length 18". The high reactance. transformer 12 within the casing was 9 /2" long, 2 wide, and only 2% high. The outer yoke core legs 21, 22 were formed from laminations .025 thick having half the width of the center leg and stacked, toaheight'of' 1%{'. The winding core leg 20 consisted of; 21.1%? stackof laminationsfi f' wide. Each bridgedgap. 26-, 27 wasslocated 2% from an opposite end of thewindingzcore leg.and was.668" long-and 4;" wide. The primary-.winding P had- 365 turns of number 20. copper wire, the heaterwindings H and H had 13 turns. of number 20 copper wire while heater winding H had 13 turns of number 20 copper wire. The secondary winding hada total of 1,672 turns of number 22 copper wirewith 836 turns in each winding section S and S Running capacitor 13 had a capacitance of 4.45 microfarads, startingv capacitor 14 had a capacitance of 0.075 microfrad. The open circuit voltage developed by the autotransformer 12 was. 532 volts. Lamp currentwasl lfi .amperes for lamp 31 and 1.0 amperes-for 1a1'np32 withan. input powerfactor of 91% Although formost applications it has been found'pref-. erable toemploy asymmetrical transformer construction; in which. the secondary winding sections S and S provide.substantially-identical leakage reactance, it will be appreciated that for some-applications a non-symmetrical construction may be employed, and that the leakage reactance of one secondary winding section may be some-. what smaller or greater than the other secondary winding section. As is well understood by those skilled in the art, the. leakage reactanceof a transformer winding may be altered by its position on, the core relative to the primary winding, or by its traverse length on the core, or by the use of magnetic shunts between the primary and secondary-windings, or by the proximity of the central and yoke coreslegsr in the region of the coreupon which the secondary winding is located. Most of the benefits of the invention are lost, however, if the transformer is con-. structed so that one of the secondary winding sections 8; or .S provides less than 40 of the total leakage react'anceofjthe transformer, and the term balancedleak age reactances', asju'sea in this application-is intended to meantwo le'akagefreactances in which neither reactan ceisless than 40% of theirftotal. Although we have shown a, particular embodiment of the invention, many modifiea; tions 'may beinade, and we intend by the appended claims to'l'cover all: such modificationsas fall within the true spiritand scope bf the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. Ballasting apparatus for fluorescent lamps comprising a high reactance transformer having a magnetic core including an elongated winding core leg and a yoke core leg completing a magnetic circuit with said winding core leg, a primary winding and a pair of secondary winding sections on side-by-side portions of said winding core leg with said primary winding located between said secondary winding sections, said secondary winding sections having balanced leakage reactances and being connected in series voltage aiding relation with each other, said core having a pair of bridged gaps, each bridged gap in the magnetic circuit of a respective one of said secondary windings, a pair of lamp circuit output leads for energizing a lamp circuit from said apparatus, and connection means for applying the combined voltages of at least said secondary winding sections across said output leads.

2. The ballasting apparatus of claim 1 in which each of said secondary winding sections is of substantially identical construction.

3. The ballasting apparatus of claim 1 in which each half of said high reactance transformer on either side of a central transverse cross-sectional plane is of substantially identical construction.

4. The ballasting apparatus of claim 1 wherein each said bridged gap is located in said winding core leg and is surrounded by a respective one of said secondary winding sections.

5. The ballasting apparatus of claim 1 wherein said secondary winding sections are connected in voltage aiding autotransformer relation with said primary winding.

6. The ballasting apparatus of claim 1 also comprising a plurality of secondary heating windings on said winding core leg concentric with said primary winding.

7. Ballasting apparatus for hot cathode type fluorescent lamps comprising a high reactance transformer having a magnetic core including a winding core leg and a yoke core leg, a primary winding and a plurality of secondary heating windings located on a central portion of said winding core leg, a pair of input leads connected to said primary winding, a plurality of pairs of output leads respectively connected to said heating windings, a pair of secondary winding sections on said winding core leg on the opposite sides of said primary winding, said secondary winding sections having balanced leakage reactances and being connected in series with each other and in voltage aiding autotransformer relation with said primary winding, said core having a pair of bridged gaps, each bridged gap in the magnetic circuit of a respective one of said secondary windings and connection means for applying the combined voltages of said primary winding and said secondary winding sections between two of said output leads which are connected to different ones of said heating windings thereby to operate said lamps from said combined voltages.

8. The ballasting apparatus of claim 7 wherein said secondary winding sections are each of substantially identical construction.

9. The ballasting apparatus of claim 7 wherein each half of said high reactance transformer on opposite sides of a transverse central cross-sectional plane is of substantially identical construction.

10. The ballasting apparatus of claim 7 wherein said bridged gaps are located in said central winding core leg each in the vicinity of a respective one of said secondary winding sections.

11. The ballasting apparatus of claim 7 also comprising a running capacitor connected in series with said secondary winding sections.

12. The ballasting apparatus of claim 11 in which at least one of said heating windings is electrically separate from said primary winding and said secondary winding sections, and a starting capacitor is connected between one of the output leads of said one heating winding and one of the output leads of another of said heating windings.

13. Ballasting apparatus for fluorescent lamps comprising a high reactance transformer having a magnetic core including an elongated winding core leg and a pair of elongated yoke core legs completing magnetic circuits with said winding core leg on opposite sides of said winding core leg, a primary winding and a pair of secondary winding sections on side-by-side portions of said winding core leg with said primary winding located between said secondary winding sections, said secondary winding sections having balanced leakage reactances and being connected in series voltage aiding relation with each other, each half of said transformer on either side of a central tranverse plane through said core being of substantially identical construction, a pair of lamp circuit output leads for energizing a lamp circuit from said apparatus, and connection means for applying the combined voltages of at least said secondary winding sections across said output leads.

References Cited in the file of this patent UNITED STATES PATENTS 2,355,968 Gustin et al. Aug. 15, 1944 2,578,395 Brooks Dec. 11, 1951 2,644,107 Keiifer et a1 June 30, 1953 2,806,199 Sola Sept. 10, 1957 2,869,037 Brooks et al. Jan. 13, 1959 

